.large-header{
    position:relative; width:100%; overflow:hidden;
    background-size:cover; background-position:center center;
}
.circle_demo{
    position:absolute;
    height:100%; width:100%;
    top:0; left:0; z-index:0;
}
#box {
    position: absolute;
    top:0; left:0;
    z-index:2;
    height:100%; width:100%;
}
.circle_demo .main-title::before{
    content:''; width:20vw;
    height:20vw; min-width:3.5em;
    min-height:3.5em;
    background-size:cover; position:absolute;
    top:50%; left:50%;
    border-radius:50%;
    z-index:-1;
    -webkit-transform:translate3d(-50%,-50%,0);
    transform:translate3d(-50%,-50%,0);
}
@media only screen and (max-width :768px){
    .demo-1 .main-title,
    .demo-3 .main-title,
    .demo-4 .main-title{ font-size:3em }
    .circle_demo .main-title{font-size:4em}
}